Unknown gunmen seized the Americans on Friday from a private residence in the capital's southeastern Dora district, according to Iraqi officials.

A member of the Iraqi security forces mans a checkpoint on the main road from Baghdad's central Jaderiyah district following the kidnapping of three Americans. (SABAH ARAR/AFP/Getty Images)

The Iraqi government has struggled to rein in the Shi'ite militias, many of which fought the U.S. military following the 2003 invasion and have previously been accused of killing and abducting American nationals.

No U.S. citizens have been abducted in Iraq since U.S. troops withdrew from the country in 2011.

The State Department said on Sunday it was working with Iraqi authorities to locate Americans reported missing, without confirming they had been kidnapped.

Hostility between Tehran and Washington has eased in recent months with the lifting of crippling economic sanctions against Iran in return for compliance with a deal to curb its nuclear ambitions and a recent prisoner swap.